I am new to flultter and dart. I see main calls the first widget as follows with added print statements.
void main() {
print('begin ');
runApp(MyApp());
print('end');
}
I see another way is
void main() => runApp(MyApp());
but when I try following it does not work
void main() => { print(' begin '); runApp(MyApp()); };
My question is if I want to run multiple statements in second( => ) approach, how I can do that and what is the name of => operator ?
i dont know why no one answered but yes its possible to call many statements with an arrow function but you need a comma in between (simlair to passing arguments to function) this can be usefull when passing many function to onTap or OnClick function
your exemple:
void main() => { print(' begin '), runApp(MyApp()) ,print(' end ')};
The => is not an operator, per se. It is a syntax that allows you to write one-liner functions that perform a single action and can also return a value. For example:
// Return a value
String getString() => 'a string';
// Do something
void doSomething() => doSomethingElse();
// It also works for getters
int _privateValue;
int get publicValue => _privateValue;
// They are also common in higher order functions
var numbers = [1, 2, 3, 4, 5];
var oddNumbers = numbers.where((n) => n % 2 != 0); // Output: [1, 3, 5]
Each of these examples is equivalent to a fully written-out method with curly brackets and return statements.
However, one-liner functions are, by definition, required to only have a single line of code. If you want a second line, you can't use a one-liner function and instead have to write a complete method:
// Incorrect, will cause the second statement to execute separately
// or will throw an error, depending on where and how you do this
void oneLinerFunction() => print('1'); print('2');
// The correct way to define a method with more than one line of code in it
void fullMethod() {
print('1');
print('2');
}
As far as your final example, combining => and {} doesn't make the one-liner function execute multiple lines. It makes the function attempt to return a map or a set, since that's what values within curly brackets means. For example:
Map<String, int> getMap() => { 'a': 1, 'b': 2 };
This will not work if you try to use it to sneak in multiple lines of code to the function. The compiler will interpret it as a set and will not work as you expect at best and throw an error at worst.
The => operator is referred to as the Arrow Operator
Functions defined inline with the => operator are referred to as Lambda Functions or Arrow Functions. These functions execute and return a single statement.
If you want to perform multiple statements, you can use Anonymous Functions. Specified with {} and without the => operator:
someFunc( () { print("1"); print("2"); print("3");} )
(Anonymous functions can have return values as well)
There's a big thread about it here on Stack Overflow
As others have said, => operator is referred to as the Arrow Operator and it is meant to be a one liner.
I came here wanting to have multiple lines of code in the onPressed of a button and this is one way to do it:
ElevatedButton(
onPressed: () {
print('one');
print('two');
},
child: Text("Click me"),
),
If you have a method that only contains one line, you can use => to save room and make the entire method declaration and body fit on one line. You can have multiple operations, as long as they fit on that single line. It basically translates to 'just return this value' (and it acts like a return statement).
void main() => runApp(MyApp());
is completely equivalent to saying:
void main(){
return runApp(MyApp());
}
However, if you have a method that includes multiple lines, you need to use brackets.
